If September is rainy, begin raking leaves as they fall and grass clippings as you mow; otherwise they'll form mucky hide-outs for pests. If needed, thatch now, followed by fertilizing and overseeding. Overseed bare patches in an established lawn will make your grass more lush, and moss and weeds won't have as much room to take over. If your lawn doesn't need thatching, over the next couple of months you can apply a fall or winter fertilizer to encourage nice green grass and healthy root development.
Fall is an ideal time to shop for and plant new trees and shrubs. They'll have a chance to establish roots over the winter, and at nurseries you'll see the beginnings of true fall colors. Reduce watering for established shrubs and trees so they can harden off in preparation for winter. Watch fruit trees for signs of mildew. Now is also the time to take cuttings from roses.
Pick tomatoes if the weather is cooling down, and let them finish ripening indoors. You can now harvest carrots, corn and potatoes. The root vegetables can be harvested for months to come, but corn isn't as forgiving of cold.
Inside your home look around your doors and windows: Is the weatherstripping torn or missing? This can become expensive if ignored. On doors, make sure the bottom seal is working properly. There are many sweeps, gaskets and thresholds designed to seal this gap. Doors generally need weatherstripping in their jambs as well. Adhesive-backed foam pads are easy to install for this purpose. Newer, energy-efficient windows generally don't require added weatherstripping, but if your windows are older, weatherstripping can keep drafts at bay and energy costs down.
Think of caulk as weatherstripping in a tube. Any gap on the outside of your home can be a candidate for caulking. Look at transition spots: corners, windows, doors, areas where masonry joins siding, or places where vents and other objects protrude from walls. Carefully read manufacturer's directions to make sure the caulk you buy will work where you plan to use it.
September inspires nesting in mice as well as humans. Mice are looking for a winter home now, and that newly insulated attic would be just the spot. Mice can squeeze through 1/4-inch openings; rats need a half-inch. Make sure all exterior vents are screened, and that there are no gaps underneath garage doors.
Check your gutters and roof. Do a quick visual check to make sure gutters are clear and secure. They'll be performing double duty soon with rainstorms and falling leaves. While you are up there, take a look at the roof as well. Look for broken or missing shingles, missing or damaged flashing and seals around vent pipes and chimneys, and damage to boards along the eaves. Pay close attention to valleys and flashings. Many leaks originate in these spots. Some patches and roofing cement now can prevent thousands of dollars of water damage later in the winter.